The present invention relates generally to air conditioning systems for motor vehicles and more specifically to an improved control therefore.
These air conditioning systems generally include a compressor, fan, condenser, evaporator and heating element and have an evaluation unit. The evaluation unit is supplied with values recorded by sensors arranged in the automobile for at least the interior temperature T.sub.i, the outside temperature T.sub.a, and the temperature after the evaporator. The evaluation unit then controls the interior temperature based on these readings.
A process of this kind is known for example, from German Application No. 2,126,599.
In air conditioning systems for motor vehicles consisting essentially of a cooling and heating device, a distinction is to be drawn between two different control devices, namely automatic temperature control, automatic climate control.
In the case of automatic temperature control, the vehicle passenger has two options for maintaining a constant interior temperature by keyboard selection options. In the first selection option for the air conditioning system (=normal position), the air conditioning system switches on only if the temperature of the outside air is not sufficient to maintain the set nominal interior temperature. However, the windows of the vehicle can mist over on the inside when the outside air is very humid and no cooling is required from the point of view of temperature level. In order to avoid this misting, the second selection option offers the "reheat" function. The evaporator of the air conditioning system is always maintained at the lowest possible temperature, but above 0.degree. C., so that the outside air supplied is dried by means of the cooling. In the subsequent heating heat exchanger, also known as counter-heating device, the cooled and dried air is reheated to the required ventilation temperature, so that a misting of the windows is avoided by the supply of dry outside air. However, since the compressor is also in operation when the temperature level of the outside air would not require a cooling, power is unnecessarily supplied by the vehicle engine for the compressor. In addition, dried air is continually supplied to the vehicle interior, so that the required minimum air humidity is not present in the interior.
The same also applies to the automatic climate control, which contains the abovementioned "reheat" function and further includes an automatic air supply and air amount control.
Independently of this, it is known in German Patent Specification No. 3,225,622, to control the air humidity in a closed space with an automatic air humidifier such that the actual value of the relative air humidity is measured, compared with a nominal value and the divergence is corrected by means of a control circuit.
In addition, a device for the control of a climate test chamber is known from German Auslegeschrift No. 1,573,368, in which both the air humidity and the air temperature from sensors arranged in the chamber are recorded and programmed changing climate investigations can be performed ovr periods of time of optional length.
Furthermore, a process for measuring the air humidity content is known from French Published Patent Application No. 2,535,083, in which a characteristic curve behavior, changing over time, of a hydrometer is compensated by correcting a characteristic curve stored in a microprocessor to maintain the measuring accuracy and consequently the accuracy of a control.
Particularly to avoid a misting of automobile windows, a device is known from German Auslegeschrift No. 2,405,230, in which a condensation of humidity on the surface of the window is determined from a change of resistance between two detector electrodes. In the case of falling below a predetermined resistance value, a detector and control circuit activates a window heating unit until, due to the window drying, the impedance between the detector electrodes has increased again so far that a signal to switch off the window heating is generated by the detector and control circuit.
The object of the invention is to further develop, with respect to control techniques a congeneric air conditioning system for the control of the temperature of the interior air such that a misting of the windows is avoided, but nevertheless a required minimum air humidity is maintained in the interior of the vehicle and the compressor is switched on and off automatically only when required.
These and other objects are provided by a process and processor which includes a further sensor for receiving the relative humidity of the outside air .phi.a and supplying it to the evaluation unit. The evaluation unit forms the difference between the inside and outside temperature (t.sub.i -t.sub.a). The evaluation unit generates a first signal when the outside humidity .phi.a at the difference of inside to outside temperature exceeds an associated characteristic value .phi.as from a characteristic curve of .phi.as as a function of the difference of temperatures stored in a memory unit. In response to the first signal, a blow-out temperature t.sub.vSoll which is a function of the inside temperature is read from storage for the recorded value of the inside temperature. The compressor is switched on when the evaporator blow-out temperature is greater than the nominal evaporator blow-out temperature from the characteristic curve at the recorded inside temperature value. the characteristic curve for the relative humidity of the outside air is lowered if there are more people in the car, as measured by signals from the seat sensors. Also, the characteristic curve of the outside temperature may be raised as a function of the rotational speed of the fan. A lower fixed temperature value t.sub.ic is supplied to the evaluation unit instead of the recorded current value of the interior temperature T.sub.i when an recirculating-air-flap is deliberately actuated for the recirculating-air operating state. Based on the value of the relative humidity of the outside air .phi.a, a third memory unit provides a signal to displace the operating state of the recirculating flap more in the direction of fresh air or recirculating air for a defined period of time. The recorded relative humidity of the outside air .phi.a is compensated as a function of the values recorded for the outside air temperature. In response to a signal dependent upon the nominal value of the interior temperature, generally set by the occupant, the characteristic curve of the relative outside humidity is raised or lowered within certain limits.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.